WO 2006/025849 A2 discloses a radiation source for high radiation power levels which includes a semiconductor diode laser as a pump radiation source and a reflector which has a grating structure. The reflector is optically connected to the semiconductor diode laser and is orientated with respect to an output beam of the diode laser in such a manner that a portion of the radiation in the output beam is reflected back into the diode laser by the reflector.
The laser radiation (transmitted by the reflector) which is produced by means of the radiation source described in WO 2006/025849 A2 may be used as pump radiation for pumping a laser-active medium. Such a laser-active medium typically has a very narrow absorption line, that is to say, a very narrow wavelength range, in which the pump radiation is absorbed. For pumping a laser-active medium, the wavelength-dependent feedback of a portion of the pump radiation to the pump radiation source is advantageous because, in this instance, stabilization of the emitted wavelength is carried out so that the portion of the pump radiation transmitted by the reflector has a spectrum having a substantially narrower band than would be the case without such stabilization. Consequently, the pump efficiency can be increased by the reflector because the portion of the pump radiation received by the laser-active medium increases.
For efficient pumping, the path of the pump radiation through the laser-active medium should also be as long as possible in order to increase the absorption. However, long beam paths can cause, in the laser-active medium, thermal effects that result in a worsening of the beam quality of the laser radiation that is produced or intensified by the active medium. Furthermore, the beam quality of the laser radiation that is produced or intensified should generally be substantially greater than that of the pump radiation so that, with similar beam waists, the pump radiation diverges to a substantially greater extent so that a good spatial overlap of the pump radiation and laser radiation as well as an efficient energy transfer from the pump radiation to the laser radiation can be ensured only over a short distance. Furthermore, for many lasers or amplifiers long path lengths within the active medium are unfavorable for other reasons. For instance, the linear dispersion is usually disruptive for ultra-short pulsed lasers and/or the non-linearity of the medium (e.g., self-phase modulation, four-wave mixing, Raman or Brillouin scattering) is disruptive at high peak power levels. Therefore, the travel path in the laser medium is selected to be as short as possible in the case of laser crystals but also in laser glasses such as, for example, amplification fibers as a laser-active medium in fiber lasers, and the pump radiation is caused to pass through several times or pumping is carried out from several sides. The pump radiation can preferably be reflected back into itself at an end mirror, whereby a portion of the pump radiation not absorbed by the laser-active medium is reflected back to the pump radiation source so that the propagation path through the laser medium is doubled and the absorption efficiency is maximized.
However, the wavelength-stabilized pump radiation which is transmitted by the reflector also generally has, in spite of the reflector which reflects the wavelength in a narrow band, a spectral portion in a wavelength range which is not absorbed or which is absorbed only weakly by the laser-active medium. That undesirable spectral portion is reflected back by the end mirror to the pump radiation source. In particular, if the pump radiation arrangement is operated at high radiation power levels, for example, in the case of three-level or quasi-three-level laser media, the absorption attenuates as a result of the increasing inversion of the laser medium, which occurs particularly in the so-called zero phonon absorption line (for example, 969 nm in Yb:YAG or 976 nm in Yb:Lu203). However, the undesirable spectral portion reflected back results in a destabilization of the pump radiation source so that the spectrum of the pump radiation which is transmitted by the reflector is expanded in an undesirable manner. Such an expansion of the pump radiation spectrum relative to the absorption spectrum of the laser-active medium reduces the efficiency of the pump radiation arrangement in spite of the reflector so that the output power of the laser or amplifier cannot be increased as desired.